Laser welding apparatus and laser welding method

ABSTRACT

A laser welding apparatus generates laser by a laser oscillator, converges the laser by a condenser lens, and applies the laser to an upper sheet and a lower sheet superposed together so as to weld the upper sheet and the lower sheet to each other. According to this apparatus, by laser irradiation, a melt pool Y is formed in the upper sheet and the lower sheet superposed together. Furthermore, by laser irradiation, the melt pool Y is caused to flow, and the upper sheet and the lower sheet are welded together.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a technology concerning a laser weldingapparatus and a laser welding method.

2. Description of Related Art

The laser welding is a welding method in which laser generated by alaser oscillator is converged by a condenser lens, and is applied tosuperposed steel sheets so as to weld the sheets. However, in the laserwelding, the joint strength declines in the case where the sheet gapbetween steel sheets that are superposed together is large. For example,if there is an inter-sheet gap larger than or equal to 0.3 mm, the weldbead surface sometimes sinks, resulting in a reduced joint strength.Besides, if the inter-sheet gap is larger than 0.5 mm, it sometimehappens that a penetration hole forms and the joint strength declines.

In the welding of a three-dimensional shape object, for example, a motorvehicle body, etc., it is difficult to properly control the inter-sheetgap to, for example, 0.3 mm or less. Meanwhile, an inter-sheet gap ofabout 1 mm is permitted practically, in the case of manufacturing ofmotor vehicle bodies and the like. Therefore, it has become an issue toweld steel sheets with an inter-sheet gap of about 1 mm without adecline in the joint strength. Now, some measures against the reductionof the joint strength of the laser welding of a large inter-sheet gapare taken in practice.

Japanese Patent Application Publication No. 2004-025219 (JP 2004-025219A) discloses a method in which an inter-sheet gap is welded by using ashim roller. However, in the case where a motor vehicle body or the likeis welded, two or three steel sheets are sometimes welded. Therefore, inthe welding method disclosed by JP 2004-025219 A, there is a need tohighly frequently carry out the replacement of shims or the like, andtherefore the production efficiency declines.

Japanese Patent Application Publication No. 2005-131707 (JP 2005-131707A) discloses a method in which laser tack welding is performedalternately with welding sites with the use of a clamp. However, in amethod disclosed by JP 2005-131707 A, a dedicated large-size clamp isneeded for the welding of a motor vehicle body or the like. Besides, inthe welding method disclosed in the JP 2005-131707 A, when a zinc-platedsteel is to be welded, the laser tack welding results in a weldingfailure caused by plating vapor blow.

In Japanese Patent Application Publication No. 2010-023047 (JP2010-023047 A), the first laser irradiation is performed in a defocusedstate to melt the upper sheet so that a protrusion is formed toward thelower sheet side and therefore the gap is reduced, and then the secondlaser irradiation is performed to accomplish a penetrating weld.However, according to the welding method disclosed in the JP 2010-023047A, since the first laser irradiation is low-energy irradiation, thefirst laser irradiation achieves the heat conduction melting in theupper sheet instead of the keyhole melting. Therefore, the processingtime becomes long, and the productivity declines.

SUMMARY OF THE INVENTION

The invention provides a laser welding apparatus and a laser weldingmethod capable of welding a plurality of sheets without a decline in thejoint strength even in the case where there is a large inter-sheet gap.

A first aspect of the invention is a laser welding apparatus for weldinga plurality of sheets superposed together, the laser welding apparatusincluding: a laser oscillator that generates laser; and a condenser lensthat converges the laser, wherein the laser welding apparatus isconfigured to weld the plurality of sheets superposed together byapplying the laser to the plurality of sheets superposed together so asto form a melt pool in the plurality of sheets, and applying the laserto the melt pool so that the melt pool flows.

In the first aspect of the invention, the laser welding apparatus may beconfigured to cause the melt pool to flow by scanning the laser appliedto the melt pool, by driving the condenser lens.

In the first aspect of the invention, the laser welding apparatus mayfurther include a jig that is drivable while holding the plurality ofsheets, and the laser welding apparatus may be configured to cause themelt pool to flow by scanning the laser applied to the melt pool, bydriving the jig.

In the first aspect of the invention, the laser welding apparatus may beconfigured to cause the melt pool to flow in a predetermined rotationdirection about a predetermined axis that extends through the melt pool,by scanning the laser applied to the melt pool in the predeterminedrotation direction about the predetermined axis.

In the first aspect of the invention, the laser welding apparatus may beconfigured to cause the melt pool to flow by scanning the laser in thepredetermined rotation direction along an outer edge portion of the meltpool after the melt pool is caused to flow in the predetermined rotationdirection about the predetermined axis.

In the first aspect of the invention, the laser welding apparatus may beconfigured to cause the melt pool to flow in the predetermined rotationdirection about the predetermined axis that extends through the meltpool and converge toward the predetermined axis by scanning the laser inthe predetermined rotation direction about the predetermined axis sothat scanning converges to the predetermined axis.

A second aspect of the invention is a laser welding method, by which aplurality of sheets superposed together are welded, the laser weldingmethod including: generating laser by a laser oscillator; converging thelaser by a condenser lens; forming a melt pool in the plurality ofsheets superposed together by applying the laser to the plurality ofsheets; and causing the melt pool to flow by applying the laser to themelt pool.

In the second aspect of the invention, when the melt pool is caused toflow, the laser applied to the melt pool may be scanned by driving thecondenser lens.

In the second aspect of the invention, when the melt pool is caused toflow, the laser applied to the melt pool may be scanned by driving a jigthat holds the plurality of sheets.

In the second aspect of the invention, when the melt pool is caused toflow, the melt pool may be caused to flow in a predetermined rotationdirection about a predetermined axis that extends through the melt poolby scanning the laser applied to the melt pool in the predeterminedrotation direction about the predetermined axis.

In the second aspect of the invention, when the melt pool is caused toflow, the melt pool may be caused to flow and enlarge by scanning thelaser in the predetermined rotation direction along an outer edgeportion of the melt pool after causing the melt pool to flow in thepredetermined rotation direction about the predetermined axis.

In the second aspect of the invention, when the melt pool is caused toflow, the melt pool may be caused to flow in the predetermined rotationdirection about the predetermined axis so as to converge toward thepredetermined axis by scanning the laser in the predetermined rotationdirection about the predetermined axis so that scanning converges to thepredetermined axis.

According to the laser welding apparatus and the laser welding methodaccording to the invention, it is possible to weld a plurality of sheetswithout a decline in the joint strength, even if the inter-sheet gap islarge.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a construction diagram showing a construction of a laserwelding apparatus in accordance with an embodiment of the invention;

FIG. 2 is a flowchart showing a flow of a welding method in accordancewith a first embodiment;

FIGS. 3A and 3B show schematic perspective views and their correspondingsectional views each taken along line B-B, respectively, showingoperation of the welding method of the first embodiment;

FIG. 4 is a flowchart showing a flow of a welding method in accordancewith a second embodiment;

FIGS. 5A and 5B show schematic perspective views and their correspondingsectional views each taken along line B-B, respectively, showingoperation of the welding method of the second embodiment;

FIG. 6 is a flowchart showing a flow of a welding method in accordancewith a third embodiment;

FIGS. 7A and 7B show schematic perspective views and their correspondingsectional views each taken along line B-B, respectively, showingoperation of the welding method of the third embodiment;

FIG. 8 is a flowchart showing a flow of a welding method in accordancewith a fourth embodiment; and

FIGS. 9A and 9B show schematic perspective views and their correspondingsectional views each taken along line B-B, respectively, showingoperation of the welding method of the fourth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to FIG. 1, a laser welding apparatus 10 will bedescribed. The laser welding apparatus 10 is an embodiment of the laserwelding apparatus of the invention. The laser welding apparatus 10 is anapparatus that performs laser welding. The laser welding is a weldingmethod in which while laser light is applied, as a heat source, to metalpieces while focused on the metal pieces so that the metal pieces arelocally melted and solidified to join together.

In the embodiment described below, it is assumed that in order toconstruct a motor vehicle body, two superposed steel sheets, that is, anupper sheet 101 and a lower sheet 102, are welded together by the laserwelding that uses the laser welding apparatus 10. It is also assumedthat the upper sheet 101 and the lower sheet 102 superposed have aninter-sheet gap of 1 mm.

The construction of the laser welding apparatus 10 will be described.The laser welding apparatus 10 includes a laser oscillator 11, anoptical path 12, a laser irradiation head 13, and condenser lenses 14.The laser oscillator 11 generates CO₂ laser or YAG laser as a weldingheat source. The optical path 12 guides the laser generated by theoscillator to the laser irradiation head 13. The optical path 12transmits the laser by bending it via mirrors, or transmits the laser byfreely curving it through an optical fiber.

The laser irradiation head 13 applies the laser transmitted through theoptical path 12, to the upper sheet 101 and the lower sheet 102 fromabove the upper sheet 101. The laser irradiation head 13 is moved to aposition over the upper sheet 101 by a robot (not shown). The condenserlenses 14 condense the laser transmitted through the optical path 12 toan appropriate size for irradiation, and scans the laser on an object tobe welded. The condenser lenses 14 are housed within the laserirradiation head 13.

Operation of the laser welding apparatus 10 will be described. The laserwelding apparatus 10 generates laser by the laser oscillator 11, andconverges the laser generated by the laser oscillator 11 through the useof the condenser lenses 14, and applies the laser converged by thecondenser lenses 14 to the upper sheet 101 and the lower sheet 102superposed together, and thus welds the upper sheet 101 and the lowersheet 102 together.

With reference to FIG. 2 and FIG. 3, a laser welding method S100 will bedescribed. The laser welding method S100 is a first embodiment of thelaser welding method that uses the laser welding apparatus of theinvention. Incidentally, steps in FIG. 2 correspond to steps in FIGS. 3Aand 3B. Besides, FIG. 3B shows sectional views each taken along line B-Bin FIG. 3A.

With reference to FIG. 2, a flow of the laser welding method S100 willbe described. In step S 110, the laser welding apparatus 10 applies alaser beam to the upper sheet 101 and the lower sheet 102 superposedtogether, from above the upper sheet 101. At this time, the laserwelding apparatus 10 scans the applied laser beam so that a regionoccupied by the trace of irradiation with the laser beam forms arectangular shape in a plan view, whereby a melt pool Y is formed in theupper sheet 101 and the lower sheet 102 superposed together.Incidentally, although in this embodiment, the trace of irradiation withlaser forms a rectangular shape, the shape of the trace of irradiationmay also be a different shape such as a circular shape, an ellipticshape, etc.

In step S120, the laser welding apparatus 10 applies a laser beam to themelt pool Y formed as described above, and scans the laser beam so thatthe trace of irradiation with the laser beam becomes a line bead,whereby an interior of the melt pool Y formed between the upper sheet101 and the lower sheet 102 is caused to flow. Although the trace ofirradiation with laser is a line bead in this case, any other trace oflaser irradiation is appropriate as long as it is a trace of laserirradiation that causes the melt pool Y to flow.

With reference to FIG. 3, operation of the laser welding method S100will be described. In step S110, the laser welding apparatus 10 appliesa laser beam, from above, to the upper sheet 101 and the lower sheet 102superposed together so that the melt pool Y is formed in the upper sheet101 and the lower sheet 102 superposed together. In this case, it doesnot matter if a penetration hole or a separate bead forms in the formedmelt pool Y.

In step S120, the scanning of laser irradiation by the laser weldingapparatus 10 causes flow of the melt pool Y formed as described above.At this time, undulation occurs in the flowing melt pool Y. The meltpool Y in which undulation occurs is gathered due to surface tension,and forms a junction portion without a penetration hole nor a separatebead.

Effects of the laser welding method S100 will be described. In the fieldof the welding of a three-dimensional shape object, such as a motorvehicle body and the like, it has been difficult to properly manage theinter-sheet gap so that the inter-sheet gap is less than or equal to,for example, 0.3 mm. On the other hand, when a motor vehicle body or thelike is to be manufactured, an inter-sheet gap of about 1 mm has beenpermitted in practice. Therefore, it has been an issue to performwelding with an inter-sheet gap of about 1 mm without decline in thejoint strength. According to the laser welding method S100, the uppersheet 101 and the lower sheet 102 with an inter-sheet gap of 1 mm can bewelded together without decline in the joint strength.

With reference to FIGS. 4, 5A, and 5B, a laser welding method S200 willbe described. The laser welding method S200 is a second embodiment ofthe laser welding method that uses the laser welding apparatus of theinvention. Incidentally, steps in FIG. 4 correspond to steps in FIGS. 5Aand 5B. Besides, FIG. 5B shows sectional views each taken on line B-B inFIG. 5A.

With reference to FIG. 4, a flow of a laser welding method S200 will bedescribed. In step S210, the laser welding apparatus 10, using the laserirradiation head 13, applies a laser beam to the upper sheet 101 and thelower sheet 102 superposed together, from above. At this time, the laserwelding apparatus 10 scans the applied laser beam so that a regionoccupied by the trace of laser irradiation forms a circular shape in aplan view, whereby a melt pool Y is formed in the upper -sheet 101 andthe lower sheet 102 superposed together. Incidentally, although in thisembodiment, the trace of laser irradiation forms a circular shape, itsuffices that the shape of the trace of laser irradiation is aconcentric shape such as an elliptic shape, etc.

In step S220, the laser welding apparatus 10 scans the laser irradiationso as to cause flow in the melt pool Y formed in the upper sheet 101 andthe lower sheet 102. Herein, an axis that extends in the verticaldirection through a center of the melt pool Y in a plan view is definedas an axis P. In step S220, the laser irradiation is scanned so as torotate in a predetermined direction about the axis P (the direction ofan arrow R in FIGS. 5A and 5B).

With reference to FIGS. 5A and 5B, operation of the laser welding methodS200 will be described. In step S210, the laser welding apparatus 10applies a laser beam to the upper sheet 101 and the lower sheet 102superposed together, from above, so that the melt pool Y that iscircular in a plan view is formed between the upper sheet 101 and thelower sheet 102 superposed together. In this case, it does not matter ifa penetration hole or a separate bead forms in the melt pool Y formed asdescribed above.

In step S220, the scanning of the laser irradiation by the laser weldingapparatus 10 causes rotation of the melt pool Y formed as describedabove. Specifically, the scanning of the laser irradiation by the laserwelding-apparatus 10 causes the formed melt pool Y to circumferentiallyflow and be stirred. At this time, the melt pool Y that is turning flowsin a circumferential direction, so as to be formed in a bowl or mortarshape. Simultaneously, undulation occurs in the flowing melt pool Y. Themelt pool Y in which undulation occurs is gathered due to surfacetension, and forms a junction portion without a penetration hole or aseparate bead.

Effects of the laser welding method S200 will be described. According tothe laser welding method S200, the upper sheet 101 and the lower sheet102 that have an inter-sheet gap of 1 mm therebetween can be weldedtogether without decline in the joint strength.

With reference to FIGS. 6, 7A, and 7B, a laser welding method S300 willbe described. The laser welding method S300 is a third embodiment of thelaser welding method that uses the laser welding apparatus of theinvention. Incidentally, steps in FIG. 6 correspond to steps in FIGS. 7Aand 7B. Besides, FIG. 7B shows sectional views each taken on line B-B inFIG. 7A.

With reference to FIG. 6, a flow of the laser welding method S300 willbe described. The flow from step S310 to step S320 is substantially thesame as the flow from step S210 to step S220 in the second embodiment,and therefore will not be described.

In step S330, the laser welding apparatus 10 scans the laser beam alongan outer edge portion of the melt pool Y formed in a mortar shape instep S320. At this time, as in step S320, the scanning of the laserirradiation is performed so as to rotate in a predetermined direction(the direction of an arrow R1 in FIGS. 7A and 7B) about the axis P.

With reference to FIGS. 7A and 7B, operation of the laser welding methodS300 will be described. The process of step S310 to step S320 operatesin substantially the same manner as the process of step S210 to stepS220 in the second embodiment, and will not be described again.

In step S330, the scanning of the laser irradiation by the laser weldingapparatus 10 enlarges the diameter of the melt pool Y formed. At thistime, the radial expansion of the melt pool Y is performed using as abase the melt pool Y that flows wholly as a mass in step S320, andtherefore a penetration hole does not form in the melt pool Y. In thecase where it is tried to form a radially expanded melt pool Y from thebeginning, a melt pool Y with a large diameter is formed from thebeginning, and therefore the probability of formation of a penetrationhole is high. However, in step S330, the diameter of the melt pool Y isincreased by using as a base the melt pool Y that flows wholly as amass, and therefore the probability of formation of a penetration holeis low.

Effects of the laser welding method S300 will be described. According tothe laser welding method S300, the upper sheet 101 and the lower sheet102 with an inter-sheet gap of 1 mm can be welded together withoutdecline in the joint strength.

With reference to FIGS. 8, 9A, and 9B, a laser welding method S400 willbe described. The laser welding method S400 is a fourth embodiment ofthe laser welding method that uses the laser welding apparatus of theinvention. Incidentally, steps in FIG. 8 correspond to steps in FIGS. 9Aand 9B. Besides, FIG. 9B shows sectional views each taken on line B-B inFIG. 9A.

With reference to FIG. 8, a flow of the laser welding method S400 willbe described. The flow from step S410 to step S420 is substantially thesame as the flow from step S210 to step S220 in the second embodiment,and therefore will not be described.

In step S430, the laser welding apparatus 10 scans the laser beam sothat the scanning converges toward the center of the melt pool Y formedin a mortar shape in step S320. That is, the laser irradiation isscanned so as to converge toward the axis P. At this time, as in stepS320, the scanning of the laser irradiation is performed so as to rotatein a predetermined direction (the direction of an arrow R2 in FIGS. 9Aand 9B) about the axis P. In other words, the laser is scanned spirallyabout the axis P.

With reference to FIGS. 9A and 9B, operation of the laser welding methodS400 will be described. The process of steps S410 to 5420 operates insubstantially the same manner as the process of steps S210 to 5220 inthe second embodiment, and will not be described below.

In step S430, the scanning of the laser irradiation by the laser weldingapparatus 10 causes a central portion of the formed melt pool Y toextend downward. At this time, the melt pool Y flowing wholly as a massin step S420 extends downward, so that the lower sheet 102 is weldedwith higher strength.

Effects of the laser welding method S400 will be described. According tothe laser welding method S400, the upper sheet 101 and the lower sheet102 that have an inter-sheet gap of 1 mm can be welded together withoutdecline in the joint strength.

Although in the foregoing first to fourth embodiments, laser is appliedfrom above the upper sheet 101 and the lower sheet 102 (two steelsheets) superposed together, the invention is not limited to thisconstruction. Even in a construction in which laser is applied fromabove three or more steel sheets superposed together, substantially thesame effects can be achieved.

Although in the first to fourth embodiments, the object to be welded issteel sheets, this does not limit the invention. For example, in aconstruction in which the object to be welded is aluminum sheets,substantially the same effects can be achieved.

Although in the first and fourth embodiments, laser is applied fromabove the upper sheet 101 and the lower sheet 102 superposed together,the invention is not limited to this. Even in a construction in whichlaser is applied to a side surface (end surface) of the upper sheet 101and the lower sheet 102 superposed together, substantially the sameeffects can be achieved.

Incidentally, the scanning of laser may be carried out by changing thefocusing position of laser through, for example, driving the condenserlens as in the foregoing embodiments, or may also be carried out bymoving a plurality of sheets relatively to the laser through drivingjigs (not shown) that hold the plurality of sheets. That is, it sufficesthat the laser and a plurality of sheets move relatively to each otherso that a melt pool is caused to flow. However, in the case where aplurality of sheets to be welded are large so that it is hard to movethe sheets, it is preferable to scan the laser by changing the focusingposition of the laser through, for example, driving the condenserlenses.

Besides, examples of jigs for use in the invention include a clamp thatclamps a plurality of sheets so that the sheets are fixed immovablyrelative to each other, a table on which a plurality of sheets areplaced, combinations of these jigs, etc.

The invention has been described with reference to example embodimentsfor illustrative purposes only. It should be understood that thedescription is not intended to be exhaustive or to limit form of theinvention and that the invention may be adapted for use in other systemsand applications. The scope of the invention embraces variousmodifications and equivalent arrangements that may be conceived by oneskilled in the art.

The invention claimed is:
 1. A laser welding apparatus for welding aplurality of sheets adjacent to each other, comprising: a laseroscillator that generates a laser; and a condenser lens that convergesthe laser, wherein the laser welding apparatus is configured to weld theplurality of sheets together by applying the laser to a first side of afirst sheet from among the plurality of sheets so as to form a melt poolwhich penetrates completely through the plurality of sheets, andapplying the laser to the melt pool so that the melt pool flows, whereinthe laser welding apparatus is further configured to scan the laser onthe melt pool so as to cause a circumferential flow of the melt pool andto cause undulation in the flowing melt pool thereby gathering the meltpool due to surface tension generated in the melt pool, and wherein theundulation includes a portion of the melt pool extending away from thefirst sheet on a second side of the first sheet opposite the first side.2. The laser welding apparatus according to claim 1, wherein the laserwelding apparatus is configured to cause the melt pool to flow byscanning the laser applied to the melt pool, by driving the condenserlens.
 3. The laser welding apparatus according to claim 1, furthercomprising a jig that is drivable while holding the plurality of sheets,wherein the laser welding apparatus is configured to cause the melt poolto flow by scanning the laser applied to the melt pool, by driving thejig.
 4. The laser welding apparatus according to claim 1, wherein thelaser welding apparatus is configured to cause the melt pool to flow ina predetermined rotation direction about a predetermined axis thatextends through the melt pool, by scanning the laser applied to the meltpool in the predetermined rotation direction about the predeterminedaxis.
 5. The laser welding apparatus according to claim 4, wherein Thelaser welding apparatus is configured to cause the melt pool to flow andenlarge by scanning the laser in the predetermined rotation directionalong an outer edge portion of the melt pool after the melt pool iscaused to flow in the predetermined rotation direction about thepredetermined axis.
 6. The laser welding apparatus according to claim 4,wherein the laser welding apparatus is configured to cause the melt poolto flow in the predetermined rotation direction about the predeterminedaxis that extends through the melt pool and converge toward thepredetermined axis by scanning the laser in the predetermined rotationdirection about the predetermined axis so that scanning converges to thepredetermined axis.
 7. A laser welding method, by which a plurality ofsheets adjacent to each other are welded, comprising: generating a laserby a laser oscillator; converging the laser by a condenser lens; forminga melt pool which penetrates completely through at least one of theplurality of sheets by applying the laser to a first side of a firstsheet from among the plurality of sheets; and causing the melt pool toflow by applying the laser to the melt pool, wherein forming the meltpool includes scanning the laser on the melt pool so as to cause acircumferential flow of the melt pool and to cause undulation in theflowing melt pool thereby gathering the melt pool due to surface tensiongenerated in the melt pool, and wherein the forming the melt poolincludes scanning the laser on the melt pool so as to cause a centralportion of the melt pool on a second side of the first sheet oppositethe first side to extend toward a melt pool on a second sheet from amongthe plurality of sheets.
 8. The laser welding method according to claim7, wherein when the melt pool is caused to flow, the laser applied tothe melt pool is scanned by driving the condenser lens.
 9. The laserwelding method according to claim 7, wherein when the melt pool iscaused to flow, the laser applied to the melt pool is scanned by drivinga jig that holds the plurality of sheets.
 10. The laser welding methodaccording to claim 7, wherein when the melt pool is caused to flow, themelt pool is caused to flow in a predetermined rotation direction abouta predetermined axis that extends through the melt pool by scanning thelaser applied to the melt pool in the predetermined rotation directionabout the predetermined axis.
 11. The laser welding method according toclaim 10, wherein when the melt pool is caused to flow, the melt pool iscaused to flow and enlarge by scanning the laser in the predeterminedrotation direction along an outer edge portion of the melt pool aftercausing the melt pool to flow in the predetermined rotation directionabout the predetermined axis.
 12. The laser welding method according toclaim 10, wherein when the melt pool is caused to flow, the melt pool iscaused to flow in the predetermined rotation direction about thepredetermined axis so as to converge toward the predetermined axis byscanning the laser in the predetermined rotation direction about thepredetermined axis so that scanning converges to the predetermined axis.13. The laser welding apparatus according to claim 1, wherein a gap isformed between the first sheet and the second sheet from among theplurality of sheets, and the laser welding apparatus is configured toapply the laser to the plurality of sheets so the melt pool extends intothe gap.
 14. The laser welding method according to claim 7, wherein agap is formed between the first sheet and a second sheet from among theplurality of sheets, and wherein the laser is applied to the pluralityof sheets so the melt pool extends into the gap.
 15. The laser weldingapparatus according to claim 13, wherein the gap is equal to or greaterthan 0.5 millimeters.
 16. The laser welding apparatus according to claim13, wherein the first sheet and the second sheet are consecutive sheets.17. The laser welding method according to claim 14, wherein the gap isequal to or greater than 0.5 millimeters.
 18. The laser welding methodaccording to claim 14, wherein the first sheet and the second sheet areconsecutive sheets.